The present invention relates generally to the field of anticounterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authentication of documents and valuable articles by moiré patterns.
Counterfeiting of documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems. The same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, etc., that are often marketed in easy to falsify packages.
The present invention is concerned with providing a novel security element and authentication means offering enhanced security for banknotes, checks, credit cards, identity cards, travel documents, industrial packages or any other valuable articles, thus making them much more difficult to counterfeit.
Various sophisticated means have been introduced in the prior art for counterfeit prevention and for authentication of documents or valuable articles. Some of these means are clearly visible to the naked eye and are intended for the general public, while other means are hidden and only detectable by the competent authorities, or by automatic devices. Some of the already used anti-counterfeit and authentication means include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms, etc. Nevertheless, there is still an urgent need to introduce further security elements, which do not considerably increase the cost of the produced documents or goods.
Moiré effects have already been used in prior art for the authentication of documents. For example, United Kingdom Pat. No. 1,138,011 (Canadian Bank Note Company) discloses a method which relates to printing on the original document special elements which, when counterfeited by means of halftone reproduction, show a moiré pattern of high contrast. Similar methods are also applied to the prevention of digital photocopying or digital scanning of documents (for example, U.S. Pat. No. 5,018,767, inventor Wicker). In all these cases, the presence of moiré patterns indicates that the document in question is counterfeit. Other prior art methods, on the contrary, take advantage of the intentional generation of a moiré pattern whose existence, and whose precise shape, are used as a means of authenticating the document. One known method in which a moiré effect is used to make visible an image encoded on the document (as described, for example, in the section “Background” of U.S. Pat. No. 5,396,559 (McGrew)) is based on the physical presence of that image on the document as a latent image, using the technique known as “phase modulation”. In this technique, a uniform line grating or a uniform random screen of dots is printed on the document, but within the pre-defined borders of the latent image on the document the same line grating (or respectively, the same random dot-screen) is printed in a different phase, or possibly in a different orientation. For a layman, the latent image thus printed on the document is hard to distinguish from its background; but when a revealing transparency comprising an identical, but unmodulated, line grating (respec-methods are also applied to the prevention of digital photocopying or digital scanning of documents (for example, U.S. Pat. No. 5,018,767, inventor Wicker). In all these cases, the presence of moiré patterns indicates that the document in question is counterfeit. Other prior art methods, on the contrary, take advantage of the intentional generation of a moiré pattern whose existence, and whose precise shape, are used as a means of authenticating the document. One known method in which a moiré effect is used to make visible an image encoded on the document (as described, for example, in the section “Background” of U.S. Pat. No. 5,396,559 (McGrew)) is based on the physical presence of that image on the document as a latent image, using the technique known as “phase modulation”. In this technique, a uniform line grating or a uniform random screen of dots is printed on the document, but within the pre-defined borders of the latent image on the document the same line grating (or respectively, the same random dot-screen) is printed in a different phase, or possibly in a different orientation. For a layman, the latent image thus printed on the document is hard to distinguish from its background; but when a revealing transparency comprising an identical, but unmodulated, line grating (respectively, random dot-screen) is superposed on the document, thereby generating a moiré effect, the latent image pre-designed on the document becomes clearly visible, since within its pre-defined borders the moiré effect appears in a different phase than in the background. However, this previously known method has the major flaw of being simple to simulate, since the form of the latent image is physically present on the document and only filled by a different texture. A second limitation of this technique resides in the fact that there is no enlargement effect: the pattern image revealed by the superposition of the base layer and of the revealing transparency has the same size as the latent image.
In U.S. Pat. No. 5,712,731 (Drinkwater et al.) a moiré based method is disclosed which relies on a periodic 2D array of microlenses. However, this last disclosure has the disadvantage of being limited only to the case where the superposed revealing structure is a microlens array and the periodic structure on the document is a constant 2D dot-screen with identical dot-shapes replicated horizontally and vertically. Thus, in contrast to the present invention, that invention excludes the use of gratings of lines as the revealing layer, both imaged on a transparent support (e.g., film) or as a grating of cylindric microlenses. Furthermore, that invention does not allow to create, as in the present invention, a document with a base layer comprising patterns-made of varying shapes, intensities and colors.
Other moiré based methods disclosed by Amidror and Hersch in U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638 rely on the superposition of arrays of screen dots which yields a moiré intensity profile indicating the authenticity of the document. These inventions are based on specially designed 2D periodic structures, such as dot-screens (including variable intensity dot-screens such as those used in real, gray level or color halftoned images), pinhole-screens, or microlens arrays, which generate in their superposition periodic moiré intensity profiles of chosen colors and shapes (typographic characters, digits, the country emblem, etc.) whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other.
In a third invention, U.S. patent application Ser. No. 09/902,445, Amidror and Hersch disclose new methods improving their previously disclosed methods mentioned above. These new improvements make use of the theory developed in the paper “Fourier-based analysis and synthesis of moirés in the superposition of geometrically transformed periodic structures” by I. Amidror and R. D. Hersch, Journal of the Optical Society of America A, Vol. 15, 1998, pp. 1100–1113 (hereinafter, “[Amidror98]”), and in the book “The Theory of the Moiré Phenomenon” by I. Amidror, Kluwer, 2000 (hereinafter, “[Amidror00]”). According to this theory, said invention discloses how it is possible to synthesize aperiodic, geometrically transformed dot screens which in spite of being aperiodic in themselves, still generate, when they are superposed on top of one another, periodic moiré intensity profiles with undistorted elements, just like in the periodic cases disclosed by Hersch and Amidror in their previous U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. U.S. patent application Ser. No. 09/902,445 further disclosed how cases which do not yield periodic moirés can still be advantageously used for anticounterfeiting and authentication of documents and valuable articles.
In U.S. patent application Ser. No. 10/183,550 “Authentication with build-in encryption by using moiré intentsity profiles between random layers”, inventor Amidror discloses how a moiré intensity profile is generated by the superposition of two specially designed random or pseudorandom dot screens. An advantage of that invention relies in its intrinsic encryption system offered by the random number generator used for synthesizing the specially designed random dot screens.
However, the disclosures above made by inventors Hersch and Amidror (U.S. Pat. No. 6,249,588, U.S. Pat. No. 5,995,638. U.S. patent application Ser. No. 09/902,445) or Amidror (U.S. application Ser. No. 10/183,550) making use of the moiré intensity profile to authenticate documents have two drawbacks. The first drawback is due to the fact that the revealing layer is made of dot screens, i.e. of a set (2D array) of tiny dots laid out on a 2D surface. When dot screens are embodied by an opaque layer with tiny transparent dots or holes (e.g. a film with small transparent dots), only a limited amount of light is able to traverse the dot screen and the resulting moiré intensity profile is not easily visible. In these inventions, to make the moiré intensity profile clearly visible, one needs to work in transparent mode; both the revealing and the base layers need to placed in front of a light table and the base layer should be preferably printed on a partly transparent support. In reflective mode, when the revealing layer is embodied by an opaque layer with tiny transparent dots or holes, the moiré intensity profile can hardly be seen. In reflective mode, one needs to use of a microlens array as master screen. In that case, due to the light focussing capabilities of the microlenses, the moiré intensity profile becomes clearly visible. The second drawback is due to the fact that the base layer is made of a two-dimensional array of similar dots (dot screen) where each dot has a very limited space within which one or a very small number of tiny shapes such as typographic characters, digits or logos must be placed. This space is limited by the 2D frequency of the dot screen, i.e. by its two period vectors. The higher the 2D frequency, the less space there is for placing the tiny shapes which, when superposed with a 2D circular dot screen as revealing layer, produce as 2D moiré an enlargement of these tiny shapes. Nevertheless, high enough frequencies are needed to ensure a good protection against counterfeiting attempts.
The present disclosure is based on the discovery that a band grating incorporating original shapes superposed with a revealing line grating yields a band moiré comprising moiré shapes which are a linear or possibly non-linear transformation of the original shapes incorporated into the band grating. Since band moiré have a much better light efficiency than moiré intensity profiles relying on dots screens, the present invention can be advantageously used in all case where the previous disclosures fail to show strong enough moiré patterns. In particular, the base band grating incorporating the original pattern shapes may be printed on a reflective support and the revealing line screen may simply be a film with thin transparent lines. Due to the high light efficiency of the revealing line screen, the strong band moiré patterns representing the transformed original band patterns are clearly revealed. A further advantage of the present invention resides in the fact that the produced moiré may comprise a large number of patterns, for example a text sentence (several words) or a paragraph of text.
It should be stressed that the present invention completely differs from the above mentioned technique of phase modulation (U.S. Pat. No. 5,396,559, McGrew) since in the present invention no latent image is present on the document and since the resulting band moiré is a transformation of the original pattern shapes embedded within the base band grating. This transformation comprises always a scaling transformation (enlargement), and possibly a mirroring, a shearing and/or a bending transformation.
Let us also note that the properties of the moiré produced by the superposition of two line gratings are well known (see for example K. Patorski, The moiré Fringe Technique, Elsevier 1993, pp. 14–16). Moiré fringes (moiré lines) produced by the superposition of two line gratings (i.e. set of lines) are exploited for example for the authentication of banknotes as disclosed in U.S. Pat. No. 6,273,473, Self-verifying security documents, inventors Taylor et al.
In the present invention, instead of using a line grating as base layer, we use as base layer a band grating incorporating original patterns of varying shapes, sizes, intensities and possibly colors. Instead of obtaining simple moiré fringes (moiré lines) when superposing the base layer and the revealing line grating, we obtain band moiré patterns which are enlarged and transformed instances of the original band patterns.
It should be noted that the approach on which the present invention is based further differs from prior methods relying on the moiré intensity profile by being able to compute and therefore predict the generated moiré pattern image from the base band image and the parameters of the revealing layer without necessarily needing to analyze the moiré in the Fourier space.